Use of cationic permselective membranes in anodizing

ABSTRACT

IN ANODIZING OR ELECTROPLATING OF ANODIZABLE METALS SUCH AS ALUMINUM AND ALUMINUM BASE ALLOYS, EMPLOYING ELECTROLYTES WHICH CONTAIN METAL SALTS, DEPLETION OF THE METAL ION CONTENT OF THE ELECTROLYTE BY MIGRATION OF METAL IONS TO THE OPPOSITE ELECTRODE IS PREVENT BY THE USE OF A NONPOLORIZING AUXILIARY ELECTROLYTE, SUCH AS A MINERAL ACID, TO SURROUND THE OPPOSITE ELECTRODE, IN A CELL HAVING TWO COMPARTMENTS HOLDING THE RESPECTIVE ELECTROLYTES, THE ELECTROLYTES BEING SEPARATED IN THE CELL BY MEANS OF A CATIONIC PERM-SELECTIVE MEMBRANE. THE METHOD AND APPARATUS ARE ADAPTED FOR THE FORMATION OF HARD INTEGRALLY COLORED ANODIC COATINGS ON ALUMINUM, AND ALSO FOR CHROMIUM PLATING.

3,634,21 USE OF CATIONIC PERMSELECTIVE MEMBRANES IN ANODIZING H. J. COATES Jan. 11, 1972 Original Filed July .20, 1967 United States Patent Office Patented Jan. 11, 1972 Int. Cl. C23b 9/00; Billk 3/10 US. Cl. 204-56 R 9 Claims ABSTRACT OF THE DISCLOSURE In anodizing or electroplating of anodizable metals such as aluminum and aluminum base alloys, employing electrolytes which contain metal salts, depletion of the metal ion content of the electrolyte by migration of metal ions to the opposite electrode is prevented by the use of a nonpolarizing auxiliary electrolyte, such as a mineral acid, to surround the opposite electrode, in a cell having two compartments holding the respective electrolytes, the electrolytes being separated in the cell by means of a cationic perm-selective membrane. The method and apparatus are adapted for the formation of hard integrally colored anodic coatings on aluminum, and also for chromium plating.

This application is a division application of application Ser. No. 654,867, filed July 20, 1967, now abandoned.

BACKGROUND OF THE INVENTION In the anodizing of aluminum, aluminum base alloys, and other anodizable metals from anodizing electrolytes which contain metal salt components, depletion of the metal salt content of the electrolyte takes place owing to migration of the metal ions to the cathode, where reduction of such ions takes place. The depleted anodizing electrolyte must then be regenerated by involved and expensive chemical or electrolytic methods. Where the reduced ion forms an insoluble compound With the anionic components of the bath, an additional problem of sludge formation is present.

A somewhat similar problem is found in the use of electroplating baths of reactive character, such as, for example, chromic acid baths, which tend to react with the metal of the anode, such as for example, lead, to form insoluble chromates. This phenomenon is observed in the electroplating of conductive substrates generally, and particularly in the plating of aluminum and aluminum base alloys with chromium, employing lead anodes. Lead chromate sludges tend to form during plating, with attendant losses of lead anodes, and depletion of the chromic acid content of the plating bath. In addition, the formation of lead chromate results in a potential barrier or polarizing layer which increases the power consumption during plating.

Recently developed improved methods of hard anodizing of aluminum and aluminum base alloys employ aqueous anodizing electrolytes containing a mineral acid, an organic acid, and a metal salt of an organic acid. Such baths are capable of yielding integral nonfading colors which show improved resistance to crazing, corrosion, and abrasion, on aluminum surfaces. The baths are employed at ambient temperature, between about 50 and 80 F, and at a current density between about 12 and 60 amperes per square foot. The organic acids are typically either aliphatic dicarboxylic acids, such as oxalic acid, or alphahydroxy monocarboxylic acids, such as glycolic acid. The metals are typically the common metals of Groups I-B, VII-B and VIII of the periodic system, namely 1ron, nickel, cobalt, copper and manganese.

Thus, for example, in employing for hard anodizing a bath containing sulfuric acid, oxalic acid, and ferric oxalate, the ferric ions migrate to the cathode, where they are reduced to ferrous ions, combining with oxalic acid present in the bath, to form a sludge of insoluble ferrous oxalate, and causing depletion of the iron and oxalic acid content of the bath, requiring expensive and cumbersome regeneration procedures.

SUMMARY OF THE INVENTION In accordance with the present invention, there are provided a novel method and apparatus whereby the depletion of the metal ion content of anodizing baths and the formation of undesirable sludges in both anodizing and electroplating operations is eliminated, and the need for expens1ve regenerating steps is avoided.

The method of the invention, whether anodizing or electroplating, is performed either continuously, or semicontinuously or batchwise in an electrolytic cell comprising an anode and a cathode, the anode being immersed in an anodizing electrolyte, and the cathode being immersed in a mineral acid, the respective electrolytes being separated by at least one cationic permselective membrane.

The term cationic permselective membrane refers to a resinous membrane which permits the free passage of cations, while resisting the passage of anions. It customarily consists of a three-dimensional network of a waterinsoluble organic polymer which partakes of the nature of an ion exchange resin and of a filter. Such permselective membranes are well known and have been proposed in connection with the desalinization of water and the electrowinning of metals from solutions.

In accordance with the invention, the permselective membrane serves both as a shield for the electrode with which it is associated, and at the same time it divides the cell into compartments, separating the mineral acid electrolyte from the anodizing or plating bath. The mineral acid bath plays no particular role other than as an elec trochemically compatible ion carrier, and is maintained separated from the anodizing or plating bath by the membrane in such manner that the undesired ions cannot migrate into it. However, a balance must be maintained between water concentration in the cathode cell and that in the anodizing electrolyte in order to prevent osmosis. The mineral acid employed is advantageously the same as that contained in the anodizing or plating bath in order to prevent contamination in the event of leakage.

The method and apparatus of the invention will be better understood by reference to the accompanying drawings, in which:

FIG. 1 is a view in cross-section of an anodizing cell in accordance with the invention;

FIG. 2 is a plan view of the anodizing cell of FIG. 1;

FIG. 3 is a view in cross-section of an alternative embodiment of an anodizing cell; and

FIG. 4 is a view in cross-section of an additional arrangement of the membrane-electrode system.

In the embodiment shown in FIG. 1, an anodizing cell 10 is provided with an anode compartment 11, and cathode compartment 12. An article 13- to be anodized, such as an aluminum article, serves as the anode, while a cathode 14 of acid-resistant material such as graphite or lead, is located in the cathode compartment. The anode and cathode compartments are formed by a permselective membrane 15 which separates the two compartments. The anode compartment thus formed is filled with a body of anodizing electrolyte 16, such as a hard coat anodizing bath of the type previously described. The cathode compartment is provided with a body of a nonpolarizing electrolyte 17, such as a mineral acid, for example sulfuric acid. The anode and cathode, respectively, are connected to a source of current, which is advantageously, but not necessarily, a direct current source.

In FIG. 2, the anodizing cell of FIG. 1 is shown in plan view.

In the embodiment of FIG. 3, the cell is filled with the anodizing electrolyte 21, in which there is immersed the anode 22, which is positioned at the center of the cell. At opposite ends of the cell there are located a pair of cathode compartments 23, each of Which forms a separate permselective membrane cell. These membrane cells are provided with a cathode 24 and are filled with a nonpolarizing electrolyte 25 such as sulfuric acid. The membranes are substantially rectangular in cross-section, but may be of any desired shape and are made of a material inert to the electrolyte compositions, such as a synthetic resin, for example polyvinyl chloride. As shown in FIG. 4, the walls of the membrane cells are provided with membrane portions 26 which form windows sealed to the electrolytes, but with the membrane portion 26 serving to permit migration of selected ions into the membrane cell compartment. Although two membrane cells are shown in the embodiment of FIG. 3, it will be apparent that as many as desired may be employed.

The permselective membrane may be made of any resinous composition suitable for this purpose, which is permeable to cations, and impermeable to anions. Examples of suitable membrane materials include: sulfonated cross-linked polymers of styrene, sulfonated phenolaldehyde condensation products, polystyrene-divinylbenzene copolymers, divinylbenzene-olefinic carboxylic acid forming copolymers, infusible condensation polymers of alkyl aryl ethers and aldehydes, e.g. resorcinol dimethyl ether sulfonic acid with formaldehyde, all of which form water-insoluble structures in the form of matrixes linked with functional groups. The membrane material sold under the designation Nepton (Ionics, -Inc., Cambridge,

Mass.), which is a sulfonated copolymer of styrene and.

divinylbenzene reinforced with an embedded material made in accordance with US. Patent No. 2,730,768, is suitable for the purposes of the invention.

The method and apparatus of the invention are especially suitable for anodizing the surface of aluminum and aluminum base alloys to form thereon a hard integrally colored oxide coating, in a single step process, employing a three-component aqueous anodizing electrolyte consisting essentially of (a) a mineral acid, such as sulfuric, boric or hydrofluoric acid, or sulfamic acid, the preferred acid being sulfuric acid in a concentration of from about 0.05% to about 4.5% by weight; (b) from about 0.5% by weight to a percentage represented by the limit of its solubility in the electrolyte of an organic acid selected from the group consisting of an aliphatic alpha-hydroxy monocarboxylic acid, and an aliphatic dicarboxylic acid; and (c) from about 0.1% by weight to a percentage represented by the limit of its solubility in the electrolyte of a metal salt of an organic acid selected from the group consisting of an aliphatic alpha-hydroxy monocarboxylic acid and an aliphatic dicarboxylic acid. The current density employed ranges from about 12 to about 60 amperes per square foot. The anodizing temperature is between about 50 CF. and 80 F. The cathode solution is preferably sulfuric acid, ranging from about 4% to about in H 80 concentration by weight.

Examples of suitable aliphatic organic acids of the types mentioned above include glycolic acid (hydroxyacetic acid), lactic acid (alpha-hydroxypropionic acid), and malic acid (2-hydroxybutanedioic acid), oxalic, malonic, succinic and maleic acids.

Suitable metals include iron, copper, nickel, cobalt, and

manganese.

The preferred organic acid is oxalic acid, and the preferred metal salt is ferric oxalate.

4 DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples will illustrate the practice of the invention, but are not to be regarded as limiting:

Example 1.IIard anodizing of aluminum Employing a conventional anodizing cell, an aluminum plate made of alloy 5252 was immersed as the anode in an anodizing electrolyte having the composition:

Sulfuric acid-9.75 g.p.l. Oxalic acid, 2H O20 g.p.l. Ferric oxalate, 6H O g.p.l. Water1 liter together with a lead cathode. The anodizing temperature was 70 F., and the current density was 48 amperes per square foot. The voltage ranged from 25 to 35 volts direct current. By means of these electrodes, a total of 28,000 ampere-minutes was passed through the electrolyte. During the anodizing process, some ferric oxalate was reduced to insoluble ferrous oxalate, resulting in depletion of the electrolyte. The ferrous oxalate could be removed by filtration, or by oxidation to ferric oxalate by periodic additions of hydrogen peroxide to the bath.

Example 2.Membrane anodizing of aluminum Using the apparatus of FIG. 1, with 30% sulfuric acid in the cathode compartment, and the bath of Example 1 in the anode compartment, an anode of alloy 5252, and a cation permselective membrane to separate the two electrolyte bodies, it was found that the presence of the membrane shielded the cathode and prevented ferric ion from reaching it and being reduced. The cell was operated for 1000 ampere-minutes per liter with no formation of insoluble yellow ferrous oxalate taking place. Operation under conventional conditions without the membrane would have produced about 12 grams of ferrous oxalate.

Example 3.Membrane anodizing with water balance Utilizing the cell of Example 1, in order to reduce the amount of water transferred by osmosis from the anode cell to the cathode cell, an equimolar cathode-anode solution with respect to water content was prepared by calculating the molar water content of the anodizing electrolyte:

Moles per liter Making the water mole fraction 0.991 in the cathode solution, the sulfuric acid content would become 0.009 mole, which corresponds to a cathode solution containing 4.7% by weight of H 80 In anodizing, thealuminum alloy was anodized in 800 ml. of the above anodizing electrolyte for each 222 m1. of 4.7% H 50 cathode solution.

Example 4.Chromiurn plating of aluminum Using the cell of Example 1, with a sheet of aluminum alloy 5252 as the cathode, and a cation perm-selective membrane, a conventional chromic acid plating solution containing 250 g.p.l. chromic acid and 2.5 g.p.l. H SO in the cathode compartment, and 30% H 50 as the electrolyte in the anode compartment, and a lead anode, a direct current was applied initially at a voltage of 6-8 volts. No corrosion of the lead anode which normally would result in formation of lead chromate, took place. The absence of such passivating lead chromate made it possible to reduce the plating voltage to 3-4 volts with no reduction in cell efiiciency, and with elimination of lead anode losses, chromic acid losses, bath contamination resulting from lead chromate formation, and power saving.

While present preferred practices and embodiments have been described and illustrated, the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. The method of anodizing an anodizable metal which comprises passing electric current between the anodizable metal as anode immersed in an aqueous anodizing electrolyte and a conductive cathode immersed in a nonpolarizing electrolyte, said electrolytes being separated by at least one cationic permselective membrane.

2. The method of claim 1 in which the anodizable metal is aluminum.

3. Method of anodizing aluminum or an aluminum base alloy in an aqueous anodizing electrolyte containing a metal ion, which comprises passing electric current between the aluminum as anode immersed in said anodizing electrolyte and a conductive cathode immersed in a nonpolarizing electrolyte, said electrolytes being separated by at least one cationic permselective membrane.

4. The method of claim 3 in which the nonpolarizing electrolyte is an aqueous solution consisting essentially of a mineral acid.

5. The method of claim 4 in which the mineral acid is sulfuric acid.

6. Method of anodizing aluminum or an aluminum base alloy to form thereon a hard, integrally colored anodic coating, which comprises passing electric current between the aluminum as anode immersed in an aqueous hard coat anodizing electrolyte including a soluble metal salt, and a conductive cathode immersed in an aqueous mineral acid electrolyte, said electrolytes being separated by at least one cationic permselective membrane.

7. Method of anodizing aluminum or an aluminum base alloy to form on the surface thereof a hard, integrally colored anodic coating, which comprises passing electric current between the aluminum as anode immersed in an aqueous anodizing electrolyte consisting essentially of (a) from about 0.05% to about 4.5% by weight of sulfuric acid; (b) from about 0.5% by weight to a percentage represented by the limit of its solubility therein of an organic acid selected from the group consisting of aliphatic alpha-hydroxy monocarboxylic acids and aliphatic dicarboxylic acids; and (c) from about 0.1% by weight to a percentage represented by the limit of its solubility therein of a metal salt of an organic acid selected from the group consisting of aliphatic alpha-hydroxy monocarboxylic acids and aliphatic dicarboxylic acids; at a current density between about 12 and about amperes per square foot at an electrolyte temperature between about 50 and about F.; and a conductive cathode immersed in an aqueous mineral acid electrolyte, said electrolytes being separated by at least one cationic permselective membrane.

8. The method of claim 7 in which the mineral acid is sulfuric acid.

9. The method of claim 7 in which the water content of the catholyte is in equimolar relation to that of the anodizing electrolyte.

References Cited UNITED STATES PATENTS 7/1963 Beer 204295 1/1963 Juda et a1 204-296 US. Cl. X.R. 204-58, 252, 296 

